Author Topic: Stuck Idle ? (Read 2460 times)

rddcw · « **on:** October 05, 2016, 08:24:27 PM »

Back to working on my Stock 74' CB 750 K4. Just replaced the old intake manifolds with New OEM Honda. Took the bike out for a ride and after riding about a mile the idle was higher around 3000 RPM ( was around 1400 when I left the house after warming up the bike ) ? Proceeded on my ride for another 20 miles or so, then checked the idle. It was still high, so I back down the idle adjuster and brought it down a little around 2000. Completed my ride another 50 miles and when I got home bike was idling at 4000 RPM. Backed the idle adjuster all the way out and had no effect at all to bring down the idle ? Even screwed in the throttle cable adjuster at the handlebar to give the throttle a little more play and still the idle stuck at high ?

Just shooting from the hip, what do you think may be going on

I've benched synched the carbs, set the float levels with the clear tube, removed and cleaned all the jets with carb. spray cleaner, carb. slides all appear to move up and down smoothly with the linkage. Bikes timing is set statically with an analog volt/ohm meter ( didn't have timing light ). Point set correctly with feeler gauge and valves adjusted to FSM's.

Haven't pulled the gas tank yet, but was trying to get some ideas from all of you Tech Guru's.

ekpent · « **Reply #1 on:** October 06, 2016, 04:02:15 AM »

If you benched synched the carbs the slides that are adjusted with the big nut on the tops of the carbs may be set too high rendering your idle adjustment knob useless. Other then that look for cable routing or something interfering with your linkage. Even though you replaced the manifolds look for large air leaks also by spraying carb, brake or whatever stuff around that area. while running.

rddcw · « **Reply #2 on:** October 06, 2016, 08:31:40 AM »

I'll look today if I get some time. Have to pull the tank off first. You would think it would have to be somewhere in the carb. adjustment, but where ? I benched synched the carbs. as outlined on the Forum ? If it is a vacuum leak at the intake manifolds either at the cylinder head or carb. side, where else would you look for a potential vacuum leak ?

flybox1 · « **Reply #3 on:** October 06, 2016, 08:42:51 AM »

take off you airbox and look into the carb throats with the idle set screw turned all the way out.
are the slides fully seated? this is easier to see through the engine side of the carb throat due to the cutout on the intake side.

rddcw · « **Reply #4 on:** October 06, 2016, 08:59:38 AM »

Flybox1, Should be able to give you some input by the afternoon. I'll remove the airbox and peak inside. Thanks for the input

rddcw · « **Reply #5 on:** October 06, 2016, 07:51:50 PM »

Flybox1, Just finished pulling the airbox off to see where the throttle sides were positioned at. Boy did I mess up or what ? With the carb. idle adjuster screw backed all the way off, the throttle slides were raised between 1/8" and 9/64" off bottom. I even removed the throttle pull cable to make sure it wasn't pulling on the linkage and made no difference in where the throttle slides were positioned at. So did something wrong

Have to review the Correct Bench Synch Procedure again for obviously I did it wrong. I'm assuming I can do this procedure when the carbs. are mounted on the bike ? And when doing this, do the throttle cables need to be unhooked ?

Also I have the throttle cables routed around the left side of the steering neck then back thru the frame and to the carbs. That seems to be the correct path ? It did feel that maybe the pull cable when unhooked at the carb. linkage may have a little drag when moving the throttle grip back and forth ?

ekpent · « **Reply #6 on:** October 06, 2016, 08:44:01 PM »

My cables go to the left also. Since the airbox is off now it may be easier to take the carb bank back off again and do the proper bench synch on a bench. Make sure you set the slides so you still have function of your carb idle screw, ideally there should be around 2-3 threads showing above the adjuster nut on the slide shafts on your carb tops.

rddcw · « **Reply #7 on:** October 07, 2016, 03:59:41 AM »

Good to know about the cable routing, now I have to look for the Carb. Bench Synch Procedures on the Forum again ? If someone could point me in that direction, I would appreciate it

evanphi · « **Reply #8 on:** October 07, 2016, 05:41:06 AM »

Do what you did before, just start with your main idle adjuster screw in the middle, rather than at either end of its range. Then adjust the individual carbs.

rddcw · « **Reply #9 on:** October 13, 2016, 06:21:13 PM »

Benched synced the carbs. with a 1/8" drill bit as per Forum instructions. Re-attached the throttle cables, made sure I had enough play, between an 1/8" and 1/4" at the throttle grip. Vacuum synched carbs. and where pretty close, just had to fine tune #3 carb. Set the bike at idle around 1400 RPM. Took the bike for a ride and throttle "still" sticking a little high, around 3000 RPM ? It seems when I push the throttle grip forward ( closing it ), it steps the RPM down ? Re-set the idle as before, but still seems to hang up in RPM a little before I rotate the throttle grip closed ?

Am I adjusting the throttle linkages incorrectly ? I even sprayed carb. cleaner around the new intake manifolds, which made no difference.

Scott S · « **Reply #10 on:** October 13, 2016, 07:17:44 PM »

Make sure the throttle grip isn't catching/rubbing on the control housing. Are you running bar end mirrors? Make sure the grip isn't rubbing on the mirror.
Is the handle bar clean? Throttle isn't hanging up on grease or dirt on the bars, is it?

adigati · « **Reply #11 on:** October 13, 2016, 09:03:37 PM »

You might want to check the advance unit. It may be sticking and not carbs at all. Also being 43 years old the springs may be weak and not allowing the spark to retard at idle. That was what caused my k3 to do the exact same thing.

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rddcw · « **Reply #12 on:** October 14, 2016, 12:44:39 AM »

I'll pull the throttle cables and housing off and apart to inspect. May be there ? Not running bar end mirrors.

Also, on the advancer unit, can I pull the assembly apart and grease it inside ? Can I spray some lubricant on the advancer to clean and free up the movement when I remove it from the bike ?

Is there an adjustment on the decelerator cable to make sure the throttle closes ? Or is that just the throttle linkage spring ?

adigati · « **Reply #13 on:** October 15, 2016, 12:08:29 AM »

Yes you can take it apart and clean it. You should be able to advance and retard it by hand easily but feel resistance of the springs. Once it's working freely, check the springs using a timing light. Idle the bike at normal rpm, hook your light to cylinder 1 or 4. Your light should show the "F" mark next to the 1-4 mark. As you go past 2500 (or so) rpm the timing light should show advance to the advance marks. As the engine decelerates, the light should show the timing going back to the normal "F" mark again. If it doesn't , the springs are weak. You will see and hear this, no worries.

The springs are not available, if you find used ones they are old as well. You can try automotive ones, but the solution I've used several times is to carefully cut off one of the spring end loops. Then bend a new loop as best you can to resemble the one you cut off. Usually if you do it to one or both springs , check with timing light inbetween. You can get it to work as intended.

If anyone has found an alternative replacement spring ......I'd like to know.

Good luck!

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rddcw · « **Reply #14 on:** October 15, 2016, 10:50:48 PM »

Adigati, I'll try looking at the advancer as outlined.

Is there a procedure for checking and adjusting BOTH throttle cables to make sure they are working correctly ?

el pachuco · « **Reply #15 on:** October 16, 2016, 10:48:56 PM »

an 1/8th inch drill bit seems too thick to me. I used a straightened out paper clip to bench sync my carbs. You might want to start over, using that method.
There should not be more than about one screw thread sticking out of the top of your slide adjuster rods.

rddcw · « **Reply #16 on:** October 17, 2016, 05:29:48 PM »

I don't know but according to other Forum members or Tech guru's, the 1/8" drill bit evenly seems to be the bench synch bit size of choice.

Scott S · « **Reply #17 on:** October 17, 2016, 06:04:14 PM »

I use a small bit, but then turn the main idle screw until there's just a sliver of an opening at the butterflies. It doesn't take much.....and you'll have to make adjustments once the bike is running anyway.

rddcw · « **Reply #18 on:** October 17, 2016, 08:37:11 PM »

Im sure I'm doing the Bench Synch correctly. Back out the Idle screw adjuster all the way so that slides drop down completely. I had the top screw exposing 1-2 threads. Now adjust idle screw so that the 1/8" drill bit just slides under. Then adjust all slides accordingly.

Is that the right procedure ?

b52bombardier1 · « **Reply #19 on:** October 18, 2016, 05:10:26 AM »

Use some of that Bosch Distributor Lube on the advancer. It is nice and thick with good heat resistance. Almost any other lubricant on the advancer gets spun off and can foul the points.

My tube is yellow and black. Not sure if it's even available any more. Axel grease might work in small amounts.

Rick

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flybox1 · « **Reply #20 on:** October 19, 2016, 09:50:08 AM »

Quote from: rddcw on October 17, 2016, 08:37:11 PM

Im sure I'm doing the Bench Synch correctly. Back out the Idle screw adjuster all the way so that slides drop down completely. I had the top screw exposing 1-2 threads. Now adjust idle screw so that the 1/8" drill bit just slides under (the slave carb slide, if there is one). Then adjust all slides accordingly.

Is that the right procedure ?

Yes. See my highlight above.

re-check:
Timing and advance are perfect
carb slides snap closed when you release the throttle. If nothing is rubbing on your grip, and the cables slide easily when off the bike, try re-routing your cables for less resistance
vacuum sync done and all are equal - if you have dial gauges, calibrate them to one carb first.

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Author Topic: Stuck Idle ? (Read 2460 times)